Most computer system processors support a special mode of operation called system management mode (SMM). SMM provides a distinct operating environment that is transparent to operating system (OS) software. This mode is often used by original equipment manufacturers (OEMs) to perform special tasks such as system management, device, power and thermal management. Server-related reliability, availability and serviceability (RAS) functions are usually implemented using SMM. SMM is typically entered by sending a system management interrupt (SMI) message to the processor. Upon acknowledging the SMI, the processor saves the current processor context, also called the Processor Save State, to a portion of system memory that is specifically allocated to SMM, referred to as system management random access memory (SMRAM), and executes SMI handler code contained in SMRAM. When the SMI handler has completed its operations, it executes a special (valid in SMM only) resume instruction, which causes the processor to reload the saved processor context from the SMRAM and resume executing the interrupted task.
In a multiprocessor system, generally a SMI message is broadcasted to all processors. The SMI handler selects one processor, referred to as the SMM monarch, to handle the event. This processor waits until all other processors rendezvous inside SMM before handling the SMI event. Non-monarch processors stay in SMM until the monarch completes the event handling. When the SMM event has been handled, the monarch will signal the other processors to exit SMM. This synchronized entry and exit behavior is implemented to prevent any resource conflicts between the two parallel environments (OS and SMM). That is, if some processors are active in the OS environment and the rest are active in the SMM environment at the same time, it is possible that they may modify a shared resource and thereby interfere with each other's operations, causing the system to crash. In addition, certain SMM events can only be handled by a specific logical processor or a set of logical processors. A broadcast ensures that this condition is always met, since all logical processors will enter SMI.
Thus, SMI handling in a multiprocessor system is complex and can consume all system resources, preventing the handling of other useful work as while a processor is in SMM, it is not available to the operating system.